There is a hissing sound coming from behind my push button panel. With a little help from the forum I determined that it was a switch over valve (the row of valves mounted behind the push botton panel) I went to the junkyard today and pulled a few off an old 82' 300D, replaced mine and its still leaking air out of the bottom (actually it's more of a suction). When it is on heat there is no leaking or hissing sound. When i turn the blower off it starts hissing, until i turn the temp. control all the was down to cold. I'm pretty sure this is the cause of my vacuum woes. Can someone help me figure this thing out? What else should I replace?

Also, I have an extra push botton panel so i switched them out and I have the same problem.

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1985 Mercedes 300DT